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You are probably aware that some percent of disk space on an ext2/ext3 file system is reserved for root (typically 5%). As documented elsewhere this can be reduced to 1% with

tune2fs -m 1 /dev/sdX (where X = drive/partition, like /dev/sda1)

but how do you check to see what the existing reserved block percentage actually is before making the change? You can find that with

dumpe2fs -h /dev/sdX

You get a raw block count and reserved block count, from which you can calculate the percentage. In the example here you can easily see that it's currently 1%, so you won't get any more available space by setting it to 1% again.

FYI If your disks are IDE instead of SCSI, your filesystems will be /dev/hdX instead of /dev/sdX.